Dielectric Relaxation Phenomena

When a polar solvent is placed in a changing electrical field, the molecules must realign so that their dipole vectors maintain the orientation corresponding to minimum energy. Because of intermolecular forces, this process does not occur infinitely fast but on a time scale which depends on the properties of the medium and which is usually on the order of 1-100 ps. Dielectric relaxation experiments provide very useful information about molecular motion in polar liquids and the ability of the solvent molecules to respond to changing electrical conditions. 

The theory of dielectric relaxation is based on a macroscopic model which considers how the polarization of the medium changes with time. As seen earlier, the polarization is made up of an orientational and a distortional component so that one may write 

When an electrical field is applied to the system, the distortional component relaxes very quickly on a time scale corresponding to electronic motion. On the other hand, the orientational component relaxes on a time scale consistent with molecular motion. In deriving an expression for the kinetics of the relaxation process, it is assumed that the rate of relaxation is proportional to the departure of the orientational polarization from its equilibrium value, that is. 

From the fundamental laws of electrostatics, the relation between the polarization and the electrical field is 

This equation shows that the static permittivity is the appropriate value when the system is equilibrated. At very high frequencies only the distortional component of the polarization remains so that 

---

In Sections 1.7 and 4.8.3, we have studied the dielectric relaxation phenomena and dielectric spectroscopy, respectively. In dielectric spectrometry, the methodology allowed us to measure the capacity and, consequently, the real part of the complex dielectric constant. The imaginary part of the complex dielectric constant was calculated, in this case, with the help of the Kramers-Kronig... 

Non-Debye dielectric relaxation was also observed in porous silicon (PS) [25,160,161], PS has attracted much attention recently, mainly due to its interesting optical and electro-optical properties that can be utilized for device applications [164,165], So far, most of the activity in this field has focused on the intense visible photoluminescence (PL) from nano-PS and the underlying physical mechanism that is responsible for the generation of light. In addition, transport and dielectric relaxation phenomena in PS have also attracted... 

---